One of the key concepts in any responsive design is the change of viewport size. That’s because mobile viewports vary greatly from desktop viewports. To control the viewport size we traditionally use the viewport meta tag. However, the viewport meta tag, like all the worst browser developments of the last forever, isn’t W3C valid. It was originally introduced by Apple in Safari and has since been adopted by other browsers. This results in an inconsistent implementation.. Happily, the W3C have ridden to our rescue once again by introducing the @viewport CSS rule.